The present invention relates generally to foam trays used for packaging applications and, more specifically, to foam trays used in vacuum packaging operations for, e.g., food products.
Various forms of packaging, particularly for food products, employ a relatively rigid support member, such as a foam tray, in which a product is supported. Substantially all of the air is evacuated from a predetermined space surrounding the tray and product, and the product is then covered by a lid, typically in the form of a relatively flexible, transparent film. The film is bonded to the tray around the product, generally by forming a heat-seal between the film and tray, to thereby enclose the product within the resultant package. Both the film and support member generally comprise materials which form a barrier to the passage of gas therethrough so that the entire package is substantially gas-impermeable. In this manner, the package protects and extends the shelf-life of the product. Examples of this type of packaging include vacuum skin packaging (VSP) and modified-atmosphere packaging (MAP).
In vacuum skin packaging, the film is thermoformable, i.e., capable of being formed into a desired shape upon the application of heat, and is thermoformed about the product on a tray by means of heat and differential pressure. Virtually all of the air is evacuated from a predefined space around the package so that, when the film is attached to the tray about the product and the resultant package is subsequently exposed to atmospheric pressure, the film is caused to conform very closely to the contour of the packaged product. Generally, sufficient heat is applied to cause the film to bond with the tray outside the periphery of the product, either by employing a heat-activatable adhesive at the interface of the film and tray or by forming the film and tray from materials that are otherwise sealingly compatible upon the application of heat, e.g., by employing similar polymeric materials, such as polyethylenes, at the seal interface that bond to one another when heated. Alternatively, a pressure-sensitive adhesive can be used. Further details are described in, e.g., U.S. Pat. Nos. Re 30,009 (Purdue et al.), U.S. Pat. No. 5,346,735 (Logan et al.), and U.S. Pat. No. 5,770,287 (Miranda et al.), the disclosures of which are hereby incorporated herein by reference.
In modified-atmosphere packaging, a food product is generally packaged in a tray having a peripheral flange to which a lidding film is secured. Prior to securing the film to the tray, air is generally evacuated from the interior of the tray and replaced by a gas which extends the shelf-life of the packaged product. In one type of MAP, a fresh red meat or other food product is packaged in a low-oxygen environment, e.g., carbon dioxide and/or nitrogen, after evacuating all or most of the air from the package. Since fresh red meat assumes a purple color in such a low-oxygen environment, to the dislike of most consumers, the lidding film contains a gas-impermeable portion that is peelably removable from a gas-permeable portion. At retail, the gas-impermeable portion is peeled from the package so that oxygen from the ambient atmosphere can enter the package via the remaining gas-permeable portion of the lid and cause the meat to "bloom," i.e., assume a bright red color that most consumers associate with freshness. Fresh meat remains in this state for about three days. Examples of this type of modified-atmosphere packaging are disclosed in U.S. Pat. Nos. 5,686,126 and 5,779,050, the disclosures of which are hereby incorporated herein by reference.
Another type of MAP employs a high-oxygen packaging environment for fresh red meat or poultry. This package is made by first evacuating the air from in and around a product-containing, gas-impermeable tray, introducing a high-oxygen environment (i.e., higher than the concentration of oxygen found in air), and then attaching a gas-impermeable lidding film to the tray to enclose the product therein. The high-oxygen environment serves to preserve the meat (e.g., by preventing microbial growth), but generally for a shorter period of time relative to a low-oxygen MAP. With a high-oxygen MAP, however, the meat remains in a constant state of bloom, due to the continued exposure to oxygen, so that a peelable lid is not needed.
In these and other lidded-tray type vacuum packaging operations, a problem that frequently occurs when using a foam tray is that the tray is damaged when the pressure is reduced during the process of removing air from the space around the tray and product. During the packaging operation, the tray-containing product is placed into a vacuum chamber, and the pressure is reduced very rapidly, e.g., from atmospheric pressure to less than about 300 milibars (a reduction of over 700 mbars) in a time period of less than about 10 seconds. Because of this rapid reduction in pressure, a sudden pressure differential develops between the pressure within the cells of the foam and the ambient atmosphere, i.e., the pressure surrounding the tray, such that the pressure within the foam cells is over 700 mbars higher than that of the ambient atmosphere. Such pressure differential causes an immediate tendency for the gas within the cells to expand. In brittle foams such as polystyrene, or in foams having mechanical defects, e.g., inconsistent cell size, this tendency towards expansion causes the gas in the cells to escape forcefully from the weaker cells and surge out of the foam, thereby rupturing cells and damaging the structural integrity of the foam.
An additional problem caused by the sudden pressure differential between the inside and outside of the foam during vacuum packaging is the delamination of a film that may be adhered to one or both major surfaces of the foam. While certain foams are sufficiently gas-impermeable for food packaging such as, e.g., PET foams, other foams, e.g., polystyrene foams, are insufficiently gas-impermeable such that a gas-impermeable film is often adhered to a surface of the foam in order to render a tray made from the foam gas-impermeable. Alternatively or in addition, a supporting film may be adhered to a surface of the foam in order to enhance the rigidity or crack-resistance of a tray made from the foam. When a sufficient amount of gas within the foam cells expands and escapes from the foam during evacuation, any such films, particularly gas-impermeable films, adhered to the foam tray are often caused to fully or partially delaminate by the escaping gas, thereby rendering the tray, and therefore the package, unusable.
Accordingly, there is a need in the art for an improved foam tray that is more suitable for vacuum packaging operations, i.e., one with increased resistance to damage upon exposure to a reduction in ambient pressure.